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List comprehension

A list comprehension builds a new list compactly in one line: [expression for x in sequence]. You can add an if at the end to filter which elements are included.

[e(x) \text{ for } x \text{ in } s]

new list of

e(x)

for each

x

s

[e(x) \text{ for } x \text{ in } s \text{ if } P(x)]

only the

x

where

P(x)

is true

Symbols

$e(x)$	the expression computed for each $x$
$s$	the sequence looped over
$P(x)$	filter condition

Example

[i**2 for i in range(4)] gives [0, 1, 4, 9].

[i for i in range(11) if i % 3 == 0] gives 4 numbers: [0, 3, 6, 9].

Read a list comprehension as an ordinary for loop written backwards: value first, then for, then optional if.

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Part of Programming with Python: Loops and conditions.